KR102526833B1 - Elongation flange crack evaluation method, metal plate selection method, press mold design method, part shape design method, and press part manufacturing method - Google Patents

Abstract

A technique for more conveniently evaluating extension flange cracks in the shear section of a metal plate for evaluation is provided. The relationship between the deformation of the edge of the hole and the deformation gradient along the radial direction, which is obtained by performing a forming analysis of the hole expansion test on the second metal plate arbitrarily selected under predetermined forming conditions and converting the hole enlargement ratio into the deformation of the edge of the hole made of true strain. It has two or more reference strain gradient information 10c by changing molding conditions. Hole expansion molding is performed on the metal plate for evaluation under the same molding conditions as each molding condition corresponding to the at least two pieces of reference strain gradient information (10c), and at least two limit hole expansion ratios in the hole expansion limit of the metal plate for evaluation are obtained, , the formable area ARA of the metal sheet for evaluation is obtained from the at least two pieces of reference strain gradient information 10c and the limit hole enlargement rate at the hole enlargement limit of the obtained at least two holes. Expansion flange cracking in the shear end surface of the metal sheet for evaluation is evaluated by the obtained formable area (ARA).

Description

Elongation flange crack evaluation method, metal plate selection method, press mold design method, part shape design method, and press part manufacturing method

The present invention relates to a method for evaluating expansion flange cracking in a shear cross section of a metal plate in press molding, a method for selecting a metal plate, a method for designing a press mold, a method for designing a part shape, and a method for manufacturing a press part.

Higher strength of steel sheets used for automobile parts is progressing, and one of the problems in press forming of the steel sheets is expansion flange cracking. However, it is difficult to predict extension flange cracks in the shear cross section by crack prediction methods such as FLD.

Here, as described in Non-Patent Document 1, it is known that the strain gradient in the vicinity of the fracture part greatly affects the fracture limit of extension flange cracking. From this, for example, in the methods described in Patent Literatures 1 to 3, an actual test for evaluating the extension flange deformation limit of the shear cross section, which is represented by a hole expansion test of various materials, and a forming analysis thereof, elongation The relationship between the flange crack limit and the strain gradient is obtained. Then, extension flange cracking is predicted based on the relationship between the obtained extension flange crack limit and the strain gradient and the press forming analysis result.

Japanese Unexamined Patent Publication No. 2009-204427 Japanese Unexamined Patent Publication No. 2017-140653 International Publication No. 2016/002880

Eiji Iizuka et al.: Firing and Machining, 51-594 (2007), 700-705

Cracks in the shear end surface in the expansion flange forming at the time of press forming of the high-strength steel sheet are present. For this reason, in order to prevent the occurrence of extension flange cracking, it is important to evaluate extension flange cracking in the shear section of a metal sheet, particularly a high-strength steel sheet.

However, in the methods described in Patent Literatures 1 to 3, it is necessary to perform molding analysis according to actual tests every time the metal sheet to be evaluated is changed, so it takes time to acquire analysis data of the extension flange crack limit.

As described in Patent Literature 3, there is also a method of calculating the strain gradient directly from a material test. However, this method requires a special device for measuring strain distribution in material testing. In addition, since the metal plate warps at the time of conical hole enlargement molding, it is difficult to measure deformation, which is not practical.

As described above, in the conventional evaluation method, when selecting a metal plate to be used for processing, it is necessary to perform a forming analysis step for calculating a strain gradient necessary for predicting extension flange cracking for each metal plate to be evaluated. Therefore, there is a problem that it takes time to determine the index (information on the strain gradient corresponding to the metal sheet to be evaluated) for evaluating the forming limit of extension flange cracking.

The present invention has been devised to solve the above problems, and an object of the present invention is to provide a technique that enables determination of press forming conditions by more simply evaluating extension flange cracks in the shear end surface of a metal sheet.

In addition, as press forming conditions, there are determination of the shape of a press part in addition to selection of a metal plate used for press forming.

As a result of repeated studies on extension flange cracking in the shear cross section, the inventors performed two or more types of hole expansion forming analysis on one type of metal plate arbitrarily selected, and converted the hole enlargement ratio analyzed into true strain. The "relationship between the deformation of the hole edge and the strain gradient in the radial direction from the hole edge", obtained as the deformation of a hole edge, is the deformation gradient regardless of the material condition (plate condition) of the metal plate to be evaluated. The knowledge that it can be used as analysis data at the time of evaluation was acquired.

That is, the inventors determined that the relationship between the deformation of the hole edge, obtained by converting the hole enlargement factor during molding in the hole expansion molding analysis into true strain, and the deformation gradient in the radial direction from the hole edge during hole expansion molding, It was determined by the initial hole diameter of the metal plate and the shape of the forming tool for which the hole enlargement test was performed, and it was found that the mechanical properties such as material strength, plate thickness, and r value were not affected. For this reason, the inventors performed a hole expansion molding analysis on a metal sheet having one of the mechanical properties, and in the hole expansion molding analysis, the hole expansion ratio during molding was converted into a true strain, and the deformation of the edge of the hole and the hole expansion were performed. If the relation of the deformation gradient in the direction along the radial direction is obtained from the edge of the hole during molding, the hole end at the hole expansion limit necessary for predicting cracks in the extension flange is easily obtained without performing forming analysis for each metal plate made of a different material. It was found that the relationship between the deformation of the edge and the deformation gradient in the direction along the radial direction from the edge of the hole at the limit of hole expansion can be obtained.

And, in order to solve the problem, one aspect of the present invention is an extension flange crack evaluation method for evaluating extension flange cracking of a metal plate for evaluation composed of a metal plate having a shear cross section, as a metal plate selected regardless of the metal plate for evaluation , For the second metal plate having arbitrarily selected plate conditions, forming analysis of the hole enlargement test under the set forming conditions was performed, and the hole enlargement ratio was converted into the deformation of the hole edge formed of true strain, obtained by hole edge has two or more reference strain gradient information representing the relationship between deformation of the hole and the strain gradient along the radial direction from the edge of the hole by changing the molding conditions, and at least two of the two or more reference strain gradient information are assigned to the reference strain gradient information Hole expansion molding is performed on the metal sheet for evaluation under the same molding conditions as the corresponding respective molding conditions, respectively, and at least two limit hole enlargement ratios in the hole expansion limit of the metal plate for evaluation are obtained, and the at least two of the above The formable region of the metal sheet for evaluation is obtained from the reference strain gradient information and the hole expansion ratio at the limit of at least two hole expansion limits obtained above, and the shear cross-section of the metal sheet for evaluation is obtained based on the obtained formable region. It is the gist of evaluating extensional flange cracking in

Further, another embodiment of the present invention is a method for selecting a metal sheet to be formed into a press part, wherein the expansion flange cracking evaluation method of the above-mentioned one embodiment is used to evaluate expansion flange cracking in a shear cross section when formed into the press part. is evaluated, and based on the evaluation, it is a gist to select a metal plate in which expansion flange cracking does not occur in the shear end surface.

Further, another aspect of the present invention is a design method of a press mold for press-forming a metal sheet, wherein the metal sheet is press-formed by the expansion flange cracking evaluation method of the above-mentioned one aspect, the expansion flange cracking in the shear cross section is evaluated, and based on the evaluation, it is the gist of obtaining a press mold capable of suppressing expansion flange cracking in the shear end face.

Further, another aspect of the present invention is a method for designing a part shape of a press part obtained by press-forming a metal sheet, in a shear cross section when the metal sheet is press-formed by the extension flange crack evaluation method of the above-mentioned one aspect. Evaluate the extension flange cracking, and determine the shape of the component with the extension flange cracking suppressed in the shear cross section based on the evaluation as the gist.

Further, another aspect of the present invention is a method for producing a press part by press forming a metal sheet to produce a press part, wherein the metal sheet is press formed into the press part by the extension flange crack evaluation method of the above aspect. , evaluation of extension flange cracks in the shear cross section is made into a gist.

Another aspect of the present invention is a method for producing a press part by press forming a metal sheet, wherein press forming conditions are determined by the elongation flange crack evaluation method of the above aspect.

ADVANTAGE OF THE INVENTION According to the aspect of this invention, it becomes possible to acquire data for the evaluation (prediction) of extension flange cracking of a shear cross section more simply.

That is, according to the aspect of the present invention, it becomes possible to easily acquire data for predicting expansion flange cracking of, for example, the shear cross section of a metal sheet. Therefore, for example, it is possible to quickly and accurately determine whether the selection of press forming conditions such as metal plates, press molds, and component shapes used when press forming various parts such as automobile panel parts, structural and skeletal parts, etc. are appropriate. be able to predict

As a result, according to the aspect of the present invention, while being able to manufacture press parts by press molding stably, it can also greatly contribute to the reduction of the defective rate of press-formed products. In addition, it becomes possible to predict the shape of the press mold with good accuracy at the design stage, which can contribute to shortening the manufacturing period of the press mold.

1 is a diagram explaining processing of an extension flange crack evaluation method according to an embodiment based on the present invention.
2 is a diagram showing an example of preliminary information data.
Fig. 3 is a diagram explaining the processing of the extension flange crack evaluation method.
4 is a diagram showing an example of reference strain gradient information.
5 is a diagram for explaining an example of a molding tool.
Fig. 6 is a diagram explaining the processing of the possible region setting step.
Fig. 7 is a diagram explaining the processing of the possible region setting step.
8 is a diagram showing the shape of a press part R in the example.
9 is a diagram illustrating evaluation according to an example.

Next, embodiments of the present invention will be described with reference to the drawings.

(1st Embodiment)

The extension flange cracking evaluation method of the present embodiment is a step of evaluating extension flange cracking of a metal plate for evaluation made of a metal plate having a shear cross section by press forming, as shown in FIG. 1, an actual test step (3) and , a limit strain gradient calculation step (4), a possible region setting step (5), and an evaluation judgment step (6). In this specification, a "metal plate for evaluation" is also described as a "metal plate to be evaluated."

In addition, preliminary information 10 for evaluation of extension flange cracking is previously stored in database 2 (storage unit). The preliminary information 10 is acquired in advance by performing the preliminary information acquisition step (1) and stored in the database (2).

＜Preliminary information acquisition process (1)＞

In the preliminary information acquisition step (1), processing is performed on a second metal plate having the same or different plate conditions as the metal plate for evaluation, specifically, a second metal plate having an arbitrarily selected plate condition regardless of the metal plate for evaluation. do. In the preliminary information acquisition step (1), molding analysis of a hole expansion test is performed on an arbitrarily selected second metal plate under set forming conditions (hereinafter also referred to as hole expansion test conditions).

The plate condition is a characteristic condition of the metal plate that is usually set when analyzing the metal plate. Plate conditions are, for example, material strength, mechanical properties of the plate, plate thickness, and the like. There is no particular limitation on the sheet conditions of the second metal sheet to be subjected to forming analysis, but, for example, a sheet condition arbitrarily selected from among a plurality of types of sheet conditions expected as a metal sheet for evaluation may be employed. That is, the sheet conditions of the second metal sheet to be subjected to the molding analysis can be arbitrarily selected and used regardless of the sheet conditions (mechanical properties, sheet thickness, etc.) of the metal sheet for evaluation. For this reason, the sheet conditions of the second metal sheet subjected to the forming analysis are usually different from the sheet conditions of the metal sheet for evaluation.

With this forming analysis, the relationship between the hole enlargement ratio and the strain gradient along the radial direction from the hole edge is obtained. Further, in the preliminary information acquisition step (1), the hole enlargement ratio during molding analysis is converted into true strain, and the hole edge deformation is obtained. In the preliminary information acquisition step (1), a process for obtaining reference strain gradient information, which corresponds to the forming conditions set for the hole enlargement test through the above process, consisting of the relationship between the deformation of the edge of the hole and the strain gradient, is performed. do. The processing in the preliminary information acquisition step (1) is performed twice or more by changing the hole expansion test conditions. The sheet conditions of the second metal sheet subjected to molding analysis may be different for each hole expansion test condition or may be the same. As the sheet conditions of the second metal sheet, sheet conditions easy to perform forming analysis may be appropriately set.

In this way, data of two or more pieces of preliminary information 10 consisting of molding conditions (hole enlargement test conditions) and reference strain gradient information are acquired, and the data of the acquired two or more pieces of preliminary information 10 are stored in a database ( 2) Store in .

Forming conditions are expressed by two types of variables: the initial hole diameter to be formed in the metal plate and the form tool shape (punch shape) for which the hole enlargement test is performed. Two or more forming conditions in which at least one of the initial hole diameter and the shape of the forming tool are changed are used as the hole enlargement test conditions.

The data of the preliminary information 10 stored in the database 2 is in the data format shown in FIG. 2, for example. In the example shown in Fig. 2, the data of the preliminary information 10 consists of a data number 10a, a molding condition 10b specifying a hole enlargement test condition, and reference strain gradient information 10c corresponding to the molding condition. . The reference strain gradient information 10c is information of "the relationship between the hole enlargement ratio and the strain gradient along the radial direction from the edge of the hole" shown in the graph 11 as shown in FIG. 3 or FIG. 4 . The reference strain gradient information 10c is composed of, for example, a conversion formula expressing the above relationship, two or more table informations composed of each data of (hole enlargement ratio, strain gradient along the radial direction from the edge of the hole), and the like. .

Here, since the deformation gradient described above influences the deformation limit of the extension flange, it is desirable to obtain the deformation limit of the extension flange in a wide deformation gradient. For this purpose, it is preferable to obtain the reference strain gradient information 10c by variously changing the initial hole diameter of the metal plate and the size of the molding tool. Usually, in various hole expansion tests, in the case of forming a hole expansion test using a forming tool (punch) of the same shape, the smaller the initial hole diameter of the metal plate, the smaller the hole expansion rate in the same hole expansion ratio of various hole expansion tests. The deformation gradient from the edge of the hole to the radial direction of the hole at the time of molding increases. In addition, the shape of the forming tool also affects the deformation gradient, and for the initial hole diameter of the same metal plate, if the shape of the forming tool is conical (see Fig. 5(a)), the deformation gradient is large, and the shape of the forming tool is cylindrical. If the shape is (see Fig. 5(b)), the deformation gradient tends to be small.

In this embodiment, in order to improve the accuracy of the extension flange crack prediction of the shear cross section, it is preferable to perform molding analysis and molding test under as many types of molding conditions as possible. However, considering the metal plate used in the actual test, the initial hole diameter formed in the metal plate is preferably 5 mm or more and 200 mm or less. If the initial hole diameter is less than 5 mm, since the punching tool is easily deformed during punching of a metal plate, which will be described later, hole edges in a uniform sheared state cannot be obtained, and experimental accuracy is reduced. In addition, if the initial hole diameter is larger than 100 mm, it is impractical because a hole diameter of a conventional forming tool for punching also becomes large, and accordingly, equipment using the punching forming tool also becomes large. More preferably, as for the initial hole diameter of a metal plate, 10 mm or more and 50 mm or less are preferable.

A conical shape, a cylindrical shape, a shoe shape, etc. can be considered for the shape of a molding tool. Above all, the shape of the molding tool may be any shape as long as it is possible to reproduce molding conditions of various strain gradients on molding analysis and to perform real tests under the same molding conditions. As for the shape of the forming tool, it is preferable to use a conical forming tool with a punch tip angle of 60°, which is described in Japanese Industrial Standards JIS Z 2256.

In the forming analysis of hole enlargement in the preliminary information acquisition step (1), an analysis is performed in which the forming conditions determined in consideration of the above are reproduced.

As a method of forming analysis, it is preferable to use a widely adopted finite element method. However, as for the forming analysis method, any forming analysis method may be employed as long as the forming conditions can be reproduced on the analysis and the deformation of the metal sheet during forming can be obtained. Hereinafter, a case in which finite element analysis is used will be described as an example.

By molding analysis of the hole enlargement test, in any molding state, a strain distribution that is largest at the hole edge and decreases with distance from the hole edge in the radial direction is obtained. Then, a strain gradient is calculated from this strain distribution. There are various definitions of this deformation, but as the deformation to be used, a deformation that strongly reflects the deformation in the circumferential direction of the hole is preferable. Such deformations include, for example, maximum peripheral deformation or equivalent plastic deformation, but maximum peripheral deformation is preferred.

The reference strain gradient information 10c, which consists of the relationship between the deformation of the hole edge and the deformation gradient, is suppressed from dependence on the material properties of the metal plate by converting the hole enlargement ratio into true strain-converted hole edge deformation.

That is, according to the findings obtained by the present inventors, the deformation of the edge of the hole, which is converted to true strain from the hole enlargement ratio during molding in hole expansion molding analysis, and the deformation gradient in the direction along the radial direction from the edge of the hole during hole expansion molding The relationship of is determined by the initial hole diameter of the metal plate and the shape of the molding tool for performing the hole expansion test, and does not affect the mechanical properties of the material such as material strength, plate thickness, and r value. Then, according to the findings obtained by the present inventor, hole enlargement molding analysis is performed on a metal sheet having any one of the mechanical characteristics, and in the hole expansion molding analysis, the hole edge converted to true strain from the hole enlargement ratio during molding By acquiring the relationship between deformation and the deformation gradient in the direction along the radial direction from the edge of the hole during the hole expansion molding, the hole expansion limit necessary for predicting expansion flange cracking can be easily achieved without performing molding analysis of different materials each time. The relationship between the deformation of the edge of the hole and the gradient of deformation in the radial direction from the edge of the hole at the limit of hole expansion can be obtained.

An example of the graph 11 of the reference strain gradient information 10c is shown in FIG. 4 . The initial hole diameter applied in this example and the punch shape as the forming tool shape 21 are shown in Table 1, Table 2 and FIG. 5 . In Fig. 5, reference numeral 20 denotes a metal plate. In addition, the graph of the reference strain gradient information 10c is also described as a master curve.

Fig. 4(a) is a graph of reference strain gradient information 10c when analyzed under hole enlargement test conditions in which the initial hole diameter is 10 mmΦ and the forming tool shape 21 is conical. Fig. 4(b) is a graph of reference strain gradient information 10c when analyzed under hole enlargement test conditions in which the initial hole diameter is 25 mmΦ and the molding tool shape 21 is conical. Fig. 4(c) is a graph of reference strain gradient information 10c when analyzed under hole enlargement test conditions in which the initial hole diameter is 50 mmΦ and the molding tool shape 21 is conical. Fig. 4(d) is a graph of reference strain gradient information 10c when analyzed under hole enlargement test conditions in which the initial hole diameter is 25 mmΦ and the molding tool shape 21 is cylindrical. Fig. 4(e) is a graph of reference strain gradient information 10c when analyzed under hole enlargement test conditions in which the initial hole diameter is 50 mmΦ and the forming tool shape 21 is cylindrical.

In this way, by changing the molding condition of at least one of the initial hole diameter and the molding tool shape, the reference strain gradient information 10c represented by a different graph 11 is obtained.

Here, the inventor uses four types of steel grades having tensile strengths of 270 MPa, 590 MPa, 980 MPa, and 1470 MPa as metal plates, and for each metal plate, the initial hole diameter is 10 mmΦ, and the molding tool shape is conical. Reference strain gradient information (10c) when analyzed under hole expansion test conditions was obtained. In this case, it was confirmed that the reference strain gradient information 10c became the same curve at least when the strain gradient was 0.1 mm ^-1 or less. In addition, even when analyzing with only the r value different, it was confirmed that the reference strain gradient information 10c became substantially the same curve (master curve) when the strain gradient was 0.1 mm ^-1 or less. In addition, even when the plate thickness was changed between 0.5 mm and 4.0 mm and confirmed, it was confirmed that the reference strain gradient information 10c became substantially the same curve when the strain gradient was 0.1 mm ^-1 or less.

Here, during finite element analysis, it is preferable to obtain the reference strain gradient information 10c by preventing excessive deformation of a part of the finite element.

From this point of view, when the reference strain gradient information 10c is obtained, the equivalent stress-equivalent plastic strain relationship of the second metal plate used for forming analysis of the hole expansion test is obtained from the uniaxial tensile test of a metal plate having a uniform elongation of 7.5% or more. It is preferable to analyze under conditions where the material properties are soft, such as obtained. Further, it is preferable to use the equivalent stress-equivalent plastic deformation obtained from the uniaxial tensile test of the second metal sheet or an approximation thereof in the forming analysis.

＜Real test process (3)＞

In the actual test step (3), forming conditions corresponding to the first reference strain gradient information selected from a plurality of reference strain gradient information 10c stored in the database 2 (hole enlargement test conditions) and the same molding conditions, In practice, hole expansion molding is performed on a metal plate made of the same material as the metal plate for evaluation, and the limit hole enlargement rate in the hole expansion limit of the metal plate for evaluation is obtained.

<Limited strain gradient calculation process (4)>

In the limit strain gradient calculation step (4), the first standard strain gradient information stored in the database (2) and the edge of the hole at the hole enlargement limit corresponding to the limit hole enlargement ratio obtained in the actual test step (3) Based on the limiting strain, which is the deformation, a deformation gradient along the radial direction from the edge of the hole corresponding to the limiting strain is calculated. That is, in the limit strain gradient calculation step (4), reference strain gradient information (10c) corresponding to the hole enlargement test conditions adopted in the test step (3) is referred to, as shown in FIG. 3, the test step (3) The deformation gradient corresponding to the deformation of the edge of the hole obtained by converting the limiting hole enlargement ratio obtained in , is obtained, and data of (deformation of the edge of the hole, deformation gradient) at the limit strain is obtained. In the example of Fig. 3, the hole enlargement ratio of 79% in the limit strain is changed to the hole edge deformation of 0.58, and from the graph 11 of the corresponding reference strain gradient information 10c, the hole edge deformation corresponding to 0.58 is obtained. This is an example of obtaining 0.095 as the strain gradient.

The process of the above actual test process (3) and the limit strain gradient calculation process (4) are performed twice or more by changing the hole expansion test conditions stored in the database (2). In this way, two or more pieces of data corresponding to the limit strain (deformation of the hole edge and strain gradient) are acquired.

＜Possible area setting process (5)＞

In the possible region setting step (5), the data of (hole expansion test conditions, reference strain gradient information 10c) is changed and the data is changed and performed twice or more, and in two or more sets of limit strains (hole edge deformation, strain gradient) ) from a set of data (see FIG. 6), a moldable region ARA as shown in FIG. 7 is obtained. The deformation of the edge of the hole corresponds to the data of the limiting deformation.

For example, with the positions of two or more sets (deformation of the hole edge and deformation gradient) obtained in the limiting strain gradient calculation step (4) as the boundary value of the moldable region ARA, the two or more sets (hole edge deformation A line passing through deformation and deformation gradient) is defined as a forming limit line L, and a region below the forming limit line is defined as a moldable region ARA.

<Evaluation judgment process (6)>

In the evaluation judgment step (6), expansion flange cracking in the shear end surface of the metal sheet to be evaluated is evaluated according to the formable region (ARA) obtained in the possible region setting step (5). In the evaluation judgment step (6), for example, forming analysis simulating press forming to be evaluated is performed, and deformation of the edge of the metal sheet to be evaluated in the press forming analysis and the metal sheet evaluated from the edge The relationship between the deformation gradient in the inward direction is evaluated by whether or not it exists within the moldable region ARA. In the case of data within the formable area (ARA), it is predicted that cracks in the expansion flange in the shear section do not occur.

<action and others>

As described in Non-Patent Document 1, the deformation limit in extension flange molding is affected by the deformation gradient near the edge. This is because when the strain gradient increases, even if the edge reaches the strain localization condition, the condition is not met inside the edge, so that the effect of suppressing the localization of strain acts and the effect of inhibiting the growth of constriction in the region with little strain increases. am. That is, for two reasons: the expansion of the deformation limit of the material and the increase of the uniformity of the deformation distribution at the edge of the hole, as the deformation gradient increases, the deformation limit of the stretch flange forming increases.

According to the present embodiment, hole enlargement molding analysis is performed on one type of metal plate selected arbitrarily, and on the forming analysis, the deformation of the hole edge converted to the true strain from the hole enlargement ratio during molding and the hole edge in the hole enlargement ratio The relationship of the strain gradient in the direction along the radial direction is acquired in advance. Then, in the present embodiment, by using the same initial hole diameter and the same molding tool as in molding analysis, an actual hole expansion test is performed to obtain the limit hole enlargement ratio in the hole enlargement limit, From the relationship between the deformation and the deformation gradient in the radial direction from the edge of the hole at the time of forming the hole, the value of the deformation gradient at the limit of hole expansion is calculated. In the same way, from the relationship between the deformation of the hole edge at the hole expansion limit and the deformation gradient in the radial direction from the hole edge at the hole expansion limit in at least two or more hole expansion tests, the elongation The formable area (ARA) of the flange forming is determined.

Then, the obtained formable area (ARA) of the stretch flange forming and forming analysis simulating press forming were performed, and the deformation of the edge of the metal sheet for evaluation in the press forming analysis and the edge in the metal sheet for evaluation By comparing the relationship of the deformation gradient from the inside to the inside, it is predicted that expansion flange cracking is suppressed if the deformation state of the edge in the press forming analysis is within the above formable region (ARA). That is, with respect to the metal sheet for evaluation, it is possible to determine whether or not to form at the time of press forming.

When obtaining the reference strain gradient information 10c, by using the hole edge deformation converted to true strain from the hole enlargement ratio, the relationship between the hole edge deformation and the deformation gradient in the radial direction from the hole edge is It is determined from the initial hole diameter of and the shape of the forming tool for which the hole enlargement test is performed, and is not affected by mechanical properties such as material strength, plate thickness, and r value. As a result, in the present embodiment, deformation of the edge of the hole in the hole expansion limit necessary for the prediction of extension flange cracking and hole expansion limit are more convenient without performing molding analysis of different materials for each metal sheet for evaluation each time. The relation of the strain gradient in the direction along the radial direction from the edge of the hole can be acquired.

From the above, in the present embodiment, data for predicting extension flange cracking of the shear cross section can be easily obtained.

In this way, according to the present embodiment, since it becomes possible to easily acquire data for predicting extension flange cracking of the shear cross section of the metal sheet to be evaluated, when press forming various parts such as panel parts of automobiles, structural and skeletal parts, etc. It is possible to quickly and accurately predict whether the metal plate to be used is appropriately selected. As a result, according to this embodiment, while being able to perform press molding stably, it can contribute greatly also to the reduction of the defect rate of a press-formed product. In addition, it becomes possible to predict the shape of the press mold with good accuracy at the design stage, which can contribute to shortening the manufacturing period of the press mold.

(Second Embodiment)

In the second embodiment, expansion flange cracking is simply evaluated by the evaluation method for extension flange cracking described in the first embodiment, and based on the evaluation, selection of press forming conditions or design change is performed.

Examples of press forming conditions include selection of a metal plate used for press forming, selection of a molding surface of a mold used for press forming, and determination of press parts to be manufactured.

For example, in the present embodiment, when selecting a metal plate to be formed into a press part, the expansion flange crack evaluation method described in the first embodiment is used to determine the expansion flange at the shear end face when it is formed into the press part. Cracking is evaluated, and based on the evaluation, a metal plate in which no expansion flange cracking occurs in the shear end surface is selected.

Further, in the present embodiment, for example, in the design of a press mold for press-forming a metal plate, in the shear cross section when the metal plate is press-formed by the extension flange cracking evaluation method described in the first embodiment. Evaluate extension flange cracking, and based on the evaluation, obtain a press mold capable of suppressing extension flange cracking in the shear end face.

Further, in the present embodiment, for example, when designing a part shape of a press part obtained by press-forming a metal plate, by the expansion flange crack evaluation method described in the first embodiment, when the metal plate is press-formed. , Evaluate the extension flange cracking in the shear end face, and, based on the evaluation, determine the shape of the part in which the extension flange cracking in the shear end face is suppressed.

Further, in the present embodiment, for example, when manufacturing a press part in which a metal plate is press-formed to produce a press part, the metal plate is pressed into the press part by the expansion flange crack evaluation method described in the first embodiment. Evaluate expansion flange cracking in the shear cross section at the time of molding.

In addition, in the present embodiment, for example, when manufacturing a press part in which a metal plate is press formed to produce a press part, the press forming conditions are determined by the extension flange crack evaluation method described in the first embodiment. do.

According to the present embodiment, for example, it is possible to quickly determine whether selection of press forming conditions such as metal plates, press molds, and component shapes used when press forming various parts such as automobile panel parts, structural and skeletal parts, etc. are appropriate. And it becomes possible to predict with good precision.

As a result, according to the aspect of the present invention, while being able to manufacture press parts by press molding stably, it can also greatly contribute to the reduction of the defective rate of press-formed products. In addition, it becomes possible to predict the shape of the press mold with good accuracy at the design stage, which can contribute to shortening the manufacturing period of the press mold.

Example

The case where a metal plate made of the plate material shown in Table 3 was press-formed to obtain a curved press part R as shown in Fig. 8 was evaluated.

A hole enlargement test was performed on the metal plate made of the plate material shown in Table 3 under the conditions of the initial hole diameter and press tool shown in Tables 1 and 2, and the limit hole enlargement ratio as shown in Table 4 was obtained. From this critical hole enlargement ratio, the deformation of the edge of the hole in terms of true strain was obtained, and the strain gradient was obtained from the graph (master curve) shown in Fig. 4 by the method described in the present embodiment. The results are shown in Table 4.

From the deformation and deformation gradient of the hole edge shown in Table 4, the forming limit line (L) and the moldable region (ARA) were obtained, and the result was as shown in FIG. 9 .

Forming analysis simulating press forming was performed for each part shape in which the radius of curvature of the curvature along the longitudinal direction of the press part R was changed. Then, the deformation of the edge of the metal sheet to be evaluated in the press forming analysis and the deformation gradient in the inner direction from the edge of the metal sheet to be evaluated are obtained, and whether it is located within the formable region (ARA) shown in FIG. 9 It was confirmed whether From the confirmed results, it was predicted from molding analysis that extension flange cracking did not occur when the radius of curvature was 300 mm or 400 mm, and cracking occurred during extension flange when the radius of curvature was 200 mm or less.

On the other hand, as a result of actually performing a press forming test by changing the radius of curvature of the curvature along the longitudinal direction of the press part R, no extension flange cracks occurred when the radius of curvature was 300 mm or 400 mm, and the radius of curvature was 200 mm. In mm or less, cracks in the extension flange had occurred.

From the evaluation results of this Example, it can be evaluated that the curvature radius of the curvature along the longitudinal direction of the press part R needs to be 300 mm or more.

In this way, the evaluation of extension flange cracking based on the present invention coincides with the results of actual press forming, and the prediction of extension flange cracking can be evaluated with good precision by the evaluation method of extension flange cracking based on the present invention. I was able to see what could be done.

Here, the entire content of Japanese Patent Application No. 2019-047363 (filed on March 14, 2019), to which this application claims priority, constitutes a part of this disclosure by reference. Here, although the description was made with reference to a limited number of embodiments, the scope of rights is not limited thereto, and modifications of each embodiment based on the above disclosure are obvious to those skilled in the art.

One ; Preliminary information acquisition process
2 ; database
3 ; Actual test process
4 ; Limit strain gradient calculation process
5 ; Possible area setting process
6 ; Evaluation Judgment Process
10; preliminary information
10a; data number
10b; Molding conditions
10c; Reference strain gradient information
ARA; moldable area
L; molding limit line

Claims (10)

An extension flange crack evaluation method for evaluating extension flange cracking of a metal plate for evaluation made of a metal plate having a shear cross section,
Regardless of the above metal sheet for evaluation, a forming analysis of a hole expansion test was performed on a second metal sheet having arbitrarily selected sheet conditions as the selected metal sheet, and the hole enlargement ratio was set to true strain. Two or more reference strain gradient information, expressed in terms of the relationship between the deformation of the hole edge and the strain gradient along the radial direction from the hole edge, obtained by converting the edge deformation, by changing the above molding conditions,
Hole expansion molding is performed on the metal sheet for evaluation under the same molding conditions as respective molding conditions corresponding to at least two pieces of the reference strain gradient information among the two or more reference strain gradient information, respectively, so as to limit the hole expansion of the metal plate for evaluation. Find at least two limit hole enlargement ratios in
Obtaining a formable region of the metal sheet for evaluation from the at least two pieces of reference strain gradient information and a limit hole enlargement ratio in the obtained at least two hole enlargement limits,
An extension flange cracking evaluation method characterized by evaluating extension flange cracking at a shear end surface in the metal sheet for evaluation using the obtained formable region. An extension flange crack evaluation method for evaluating extension flange cracking of a metal plate for evaluation made of a metal plate having a shear cross section,
Regardless of the above metal sheet for evaluation, a forming analysis of a hole expansion test was performed on a second metal sheet having arbitrarily selected sheet conditions as the selected metal sheet, and the hole enlargement ratio was set to true strain. A storage unit for storing reference strain gradient information expressed as a relationship between deformation of the edge of the hole and deformation gradient along the radial direction from the edge of the hole obtained by converting it into deformation of the edge, linked to the molding conditions;
In the storage unit, two or more of the reference strain gradient information with different molding conditions are stored,
As a step of evaluating expansion flange cracking of the metal sheet for evaluation,
Hole expansion molding is performed on the metal plate for evaluation under the same molding conditions as those corresponding to the first reference strain gradient information selected from the plurality of reference strain gradient information stored in the storage unit, and the metal plate for evaluation is formed. An actual test step of obtaining a limit hole enlargement rate in the hole enlargement limit of
Based on the first reference strain gradient information and the limit strain, which is the deformation of the hole edge at the hole expansion limit corresponding to the limit hole enlargement ratio obtained in the actual test process, a radius from the edge of the hole corresponding to the limit strain A limit strain gradient calculation step of calculating a strain gradient along a direction;
Possibility of obtaining a moldable region from a set of two or more sets of data (the limit strain and the strain gradient) obtained by performing the actual test step and the limit strain gradient calculation step twice or more by changing the reference strain gradient information Zone setting process
to provide,
Expansion flange cracking evaluation method characterized by evaluating expansion flange cracking at a shear end surface in the metal sheet for evaluation using the formable region obtained in the possible region setting step. According to claim 1 or 2,
The sheet condition is at least one of the mechanical properties and sheet thickness of the second metal sheet. According to claim 1 or 2,
In the evaluation by the formable region, molding analysis simulating press forming is performed, and deformation of the edge of the metal sheet for evaluation in the forming analysis is performed, and inward from the edge in the metal sheet for evaluation. An expansion flange crack evaluation method characterized in that the relationship between the strain gradient of is determined by whether or not it exists within the formable region. According to claim 1 or 2,
When obtaining the reference strain gradient information, the equivalent stress-equivalent plastic strain relationship of the second metal sheet used in the forming analysis of the hole enlargement test is obtained from a uniaxial tensile test of a metal sheet having a uniform elongation of 7.5% or more, 2 Elongation flange crack evaluation method characterized by using the equivalent stress-equivalent plastic strain relationship obtained from a uniaxial tensile test of a metal sheet or an approximate formula thereof for forming analysis. As a method of selecting a metal plate to be formed into a press part,
Expansion flange cracking at the shear end surface when molded into the press part is evaluated by the expansion flange crack evaluation method according to claim 1 or 2, and based on the evaluation, the expansion flange at the shear end surface is evaluated. A method for selecting a metal plate, characterized in that for selecting a metal plate in which cracks do not occur. As a method of designing a press mold for press forming a metal plate,
Evaluate the expansion flange cracking in the shear end surface when the metal sheet is press-formed by the expansion flange crack evaluation method according to claim 1 or 2, and based on the evaluation, the expansion flange crack in the shear end surface A press mold design method characterized by obtaining a press mold capable of suppressing cracking. A method for designing a part shape of a press part obtained by press forming a metal plate,
Evaluate the expansion flange cracking in the shear end surface when the metal sheet is press-formed by the expansion flange crack evaluation method according to claim 1 or 2, and based on the evaluation, the expansion flange crack in the shear end surface A part shape design method characterized by obtaining a part shape with crack suppression. A method for producing a press part by press forming a metal plate to produce a press part,
Production of a press part characterized by evaluating expansion flange cracking in a shear end surface when the metal sheet is press-formed into the press part by the method for evaluating expansion flange cracking according to claim 1 or 2. method. A method for producing a press part by press forming a metal plate to produce a press part,
A method for producing a press part characterized by determining press forming conditions by the method for evaluating cracking of an extension flange according to claim 1 or 2.

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